The single most important factor to be considered in selecting the proper compression (top) ring face
coating material is the service requirements the engine will be operated under. Will the engine be
subjected to unusual speed or load (high temp conditions), stop and go short trip driving, occasional or
regular strip use, or in a high dust or dirt environment?
The three popular types of compression ring face coatings are cast iron, moly, and chrome. Each has
advantages of its own with respect to operating conditions.
Cast iron rings are a durable wear surface in
normal operating conditions and are less costly than the moly or chrome faced rings. For typical light duty service where the vehicle is not subjected to long periods of high speed or load operation and is run primarily on paved streets, plain cast iron is a good choice when not subjected to unusual dirt or heat conditions.
Moly rings have a very high resistance to scuff and superior oil control capability. When faced with occasional or continuous high speed and/or load conditions (where the engine is subject to periods of high temperature ranges), moly is a good choice because of its scuff resistance and strength. Moly, which is an acronym for molybdenum, is quite porous in its applied state, which results in excellent retention of oil in the face of the ring. Moly also has the highest melting point of the three popular face coatings which results in its capability to live better under more severe operating conditions, or more specifically, to resist scuffing and scoring. Moly rings are better suited to performance engines in which the rings are expected to serve adequately, through-out the engines entire life.
Chrome rings have good resistance to scuff (better than iron) but do not exhibit moly's oil retention capabilities.
In a dusty environment encountered while operating on dirt or unpaved roads, chrome is the best choice. As mentioned earlier moly, because of Its porosity, holds oil on the O.D. face of the ring which helps inhibit scuffing. Yet the pores on the material also can serve as a trap for foreign materials. Because the incoming air/fuel mixture will probably contain some abrasive contaminant in a dusty environment, chrome with its smoother O.D. surface, is a better choice. Chrome has more resistance to scuffing and scoring than cast iron, but somewhat less than moly.
While it is best to use the manufacturers specifications, you can use the following formulas if your rings did not come with the ring gap specifications.
Compression Ring: .0045 X Bore (stock bore = 3.68) = .016
Oil Wiper Ring: .0030 X Bore
(stock bore = 3.68) = .011